The present invention relates to the structure of a strip line type circulator with excellent electric characteristics appropriate for mass production.
FIG. 1 (A) is a principle drawing illustrating the 3 port symmetry (Y shape) junction. The Y shaped symmetry junction is obtained after the 3 coaxial transmission lines 1a, 2a, and 3a have been converted to the strip lines and are then conjoined in a plane mutually maintaining the 120.degree. angle. At the junction, magnetic components 5 are inserted between the inner conductors 1, 2, and 3 and the earth conductor 4 which also functions as a housing. The ends of 1, 2, and 3 are shorted with the earth conductor 4. The (microwave) circulator is obtained by applying a static magnetic field to the magnetic component 5 from outside and at the same time by adding capacity to the center of or the peripheral exterior of the junction. At this level, the inner conductor 1, 2, and 3 at the center where they conjoined, are supposed to be mutually insulated.
When a high frequency input signal is applied to the terminal 1a in FIG. 1 (A), electric current i.sub.1 flows to the inner conductor 1 at the junction as illustrated in FIG. 1 (B), resulting in generation of a high frequency magnetic field h.sub.1. The magnetic component 5 mounted on the junction turns h.sub.1 by 120.degree. within the plane of the junction and generates high frequency magnetic field h.sub.3. The h.sub.3 generates high frequency current i.sub.3 in the inner conductor 3. The i.sub.3 induces a high frequency output signal to the terminal 3. On the other hand, if the functions, etc. of the magnetic component are adjusted beforehand, the direction of the composed vector h.sub.2 of the high frequency magnetic fields h.sub.1 and h.sub.3 becomes, as illustrated in FIG. 1 (B), parallel to the inner conductor 2, while the current i.sub.2 induced by h.sub.2 perpendicularly intersects the inner conductor 2, to which the current cannot flow. That is, the input signal into the terminal 1a is transferred to the terminal 3a but not to the terminal 2a. 2a is called an isolation terminal. It is necessary to adjust the external magnetic field by a magnet, etc., so that the phase of the vector of h.sub.1 and h.sub.3 may be correctly maintained as illustrated in FIG. 1 (B). And also, it is necessary to add capacitors, arranged in series or in parallel, to the terminals 1a, 2a, and 3a on the exterior of the junction. From the above description, it may be understood that the circulator function can be obtained by close interactions (without leakage) with respect to correct vector relations among the 4 components, viz. the 3 inner conductors, which are arranged at an angle of 120.degree. to one another in the plane of the junction, and the magnetic component. With respect to the vector relations in FIG. 1 (B), it is desirable that all conditions are satisfactory throughout the area at the junction, within the magnetic component, in particular. In order to attain this, it is necessary to restrain disorder, such as space distortion of the high frequency magnetic field vector within the magnetic component and its periphery as well as leakage, at the minimum level. A theoretical drawing is given in FIGS. 2A and 2B to illustrate a concentrated constant type circulator proposed to obtain a particularly small-sized circulator. The circulator in FIGS. 2A and 2B has been disclosed in Japanese Patent publication (after examination) No. 15058/66 corresponding to U.S. Pat. No. 3,286,201.
The proposition described that (1) this circulator incorporates the inductors 6, 7, and 8 as the inner conductors of the junction; (2) incorporates concentrated constant capacitors between the transmission lines and the inductors; (3) maintains the magnetic field applied to the magnetic component maintained at the level of "above resonance" function which is higher than the magnetic resonance; (4) and further, it employs Y shaped wiring as a means of connecting the 3 inductors 6, 7, and 8 (the .DELTA. shape is also described). Thus, it is equipped with all the fundamental technological requirements for a concentrated constant small sized circulator. At the same time, there was a proposition where a substrate having a conductor on the surface of the same is utilized for obtaining the inductance (for instance, Japanese Patent publication after examination No. 11290/66, and No. 11291/66).
In contrast with the inductor in FIGS. 2A and 2B, a proposition (Japanese Patent publication after examination No. 4088/67) was made, where the inner conductor of the strip line of the junction is split into two or more multiple parallel lines as illustrated in FIGS. 3(A) through 3(E). Examples of the configuration of the inner conductor are illustrated in FIGS. 3(A), 3(B), and 3(C), and the structure of the junction in FIG. 3(D). If the inner conductor is made broad or wide as indicated in FIG. 1(A), the 3 inner conductors shelter one another, while a capacity independent of circulator function is generated between the inner conductors. This is not practicable.
The most used type in the VHF, UHF bands at present is the two parallel strip line type. However, when the frequencies used in such a type are high, circulator is small-sized but requires high electric current, the two-parallel-line strip line type has 2 defects. One of the defects is related to the electrical characteristics. Examples of the inner conductor of the two-parallel-line strip line are illustrated in FIGS. 3(A), 3(B), and 3(C). If the condition of the current vector is studied, it can be noted that at the point A in FIG. 3(E), rectangular current components i.sub.1a, and i.sub.1b which are undesirable for the function of the circulator are generated. These current components generate magnetic field components h.sub.1a, h.sub.1b within the magnetic component around point A, thus deteriorating the electrical characteristics of the circulator. The disorder of the electro-magnetic field at point A grows larger as the operational frequencies become higher, which in turn increases the circulator loss, narrowing the band width. Another defect of the parallel arm circulator shown in FIGS. 3(A) through 3(F) lies in the difficulties in the assembly of the inner conductor of the junction as well as the production of the earth conductor which also doubles as the housing. In order to avoid this, a proposition (Japanese Patent publication after examination No. 19010/74) was made involving a method where printed circuit boards with through-hole connections are used, and a method (Japanese Patent publication after examination No. 12709/75) was proposed where the circular inner conductors (A), (B), etc., are used as illustrated in FIGS. 4(A) and 4(B) and assembled as depicted in FIG. 4(C). However, the former method is not practicable, in consideration of the fact that most of the circulators are used for the protection of high power transistors for VHF, UHF bands and for reducing spurious resonance resulting from non-linearity of the high power transistors. That is, the conductors of the printed circuit being very thin, their copper loss is great, resulting in an increase in heat generated therefrom and a vicious cycle begins. For this reason, a thick conductor pattern is currently used, which makes manual production imperative, and also makes the circulator one of the most costly components incorporated in communications equipment.
With respect to the latter (Japanese Patent publication after examination 12709/75) the inner conductors of the strip line of the junction are circular as depicted in FIG. 4(A) or 4(B) which are assembled as illustrated in FIG. 4(C). With this configuration, the number of terminals of the inner conductors are three for both the lines and the short circuit ends, while the structure of the junction case, serving as the earth conductor as well, and a junction which can be readily assembled are appropriate for mass production. However, based on electrical properties, generation of modes unnecessary for the function of the circulator at points A and A' cannot be prevented, since the current of the inner conductor as illustrated in FIGS. 4(D) and 4(E) is very similar to the rectangular components of current vector of FIG. 3(F) in the close vicinity of point A or point B.